In analog telephones, the frequency band of a speech signal transmitted through a telephone circuit is limited to a narrow band such as 300-3400 Hz, for example. Thus, the quality of sound of a conventional telephone circuit is not good enough. In addition, in digital speech communication such as mobile telephones, since the band is limited as in the analog circuits because of rigid limits of bit rates, the quality of sound is not good enough as well.
Recently, however, with the development of speech compression technology (speech encoding technology), radio transmission of a wide-band speech signal (such as 50-7000 Hz) at a low bit rate has become possible. However, since both transmitting end and receiving end must support a corresponding wide-band speech encoding/decoding method, and base stations on both sides must be fully equipped with a network for wide-band encoding, it has only been put to practical use in part of business communication systems. To implement it in public telephone communication networks, it will not only entail an immense economic burden, but also take a lot of time before spreading.
Accordingly, a problem of the quality of sound in the conventional analog telephone circuit communication and digital speech communication remains unsolved.
Thus, Patent Documents 1 and 2 disclose, for example, a method of generating or restoring a wide-band signal from a narrow-band signal at a receiving side in a pseudo way. A frequency band extension device of the Patent Document 1 extracts a fundamental period of speech by calculating autocorrelation coefficients of a narrow-band speech signal and obtains a wide-band speech signal from the fundamental period. In addition, a wide-band speech signal restoration device of the Patent Document 2 encodes a narrow-band speech signal through an encoding method based on analysis by synthesis, and obtains a wide-band speech signal by carrying out zero filling (oversampling) to a sound source signal or speech signal obtained as a final result of the encoding.